Parts for machine structural use, for example, gears of automatic transmissions and sheaves of continuously variable transmissions, bearings, constant velocity joints, hubs, and other power transmission parts are required to have a high surface fatigue strength. In the past, for the above parts, JIS SCr420, SCM420, and other case hardened steels with C of around 0.2% have generally been used for the material, while a hardened layer of a martensite structure with C of around 0.8% has been formed on the surface of the part by carburized quenching so as to raise the surface fatigue strength in use.
However, carburized quenching is treatment which takes a long time of 5 to 10 hours, in some cases more than 10 hours, along with the austenite transformation at the high temperature of around 950° C., so heat treatment deformation (quenching strain) due to the crystal grain coarsening unavoidably becomes greater. For this reason, parts for which a high dimensional precision has been demanded have had to be ground, honed, and otherwise finished after carburized quenching.
In recent years, there has been rising demand for reducing the noise of automobile engines etc., so surface hardening with less heat strain compared with carburized quenching, such as induction hardening and soft nitriding, have come under the spotlight.
Induction hardening heats a steel material in a short time. Since only the necessary part of the surface layer is transformed to austenite and hardened, there is little hardening strain and it is possible to obtain a surface hardened part with a high dimensional precision.
However, to obtain a hardness equivalent to that of a carburized quenched material by only induction hardening, it is necessary to add 0.8% or more of C to the steel material. The hardness of the inside of the material, which has no relation to improvement of the surface fatigue strength, also rises and remarkable deterioration of the machineability occurs. Therefore, it is not possible to just increase the amount of C in the steel material without proper consideration, so there is a limit to improving the surface fatigue strength by just induction hardening.
Soft nitriding is a surface hardening method which causes the diffusion and permeation of mainly nitrogen and carbon simultaneously at the steel material surface in the temperature region below the transformation point of about 500 to 600° C. so as to form a hardened layer and improve the wear resistance, seizing resistance, fatigue resistance, etc.
At the steel material surface, the diffused nitrogen forms nitrides in the steel, forms a compound layer comprised of mainly Fe3N, Fe4N, and other Fe nitrides at the surfacemost layer of a general steel material, and forms a nitrided layer in which N is diffused inside from the surfacemost layer of the steel material.
Soft nitriding can be performed at a low temperature. Compared with the case of carburized quenching, a short treatment time of about 2 to 4 hours is enough, so this is often applied to steel parts where low strain is required.
However, with just soft nitriding, the hardened layer depth is small, so application to a gear etc. of a transmission at which a high surface pressure is applied is difficult.
Recently, as a method for compensating for the defects in induction hardening and soft nitriding and obtaining better mechanical properties, in particular improving the surface fatigue strength, performing soft nitriding, then induction hardening is being experimented with.
PLT 1 proposes a method of production which combines gas soft nitriding and induction hardening so as to make up for their individual defects and improve the softening resistance and obtain superior mechanical properties, particularly high surface fatigue strength.
The method of production of PLT 1 treats a steel material by gas soft nitriding to form a compound layer, then treats this by induction hardening to break up and diffuse into the steel the nitrogen compounds in the compound layer which is formed by the gas soft nitriding so as to form a hardened nitrided layer.
Note that, in the following explanation, the layer which is comprised of Fe3N, Fe4N, and other Fe nitrides which are formed at the surfacemost layer of the steel material by soft nitriding will be referred to as the “compound layer”, while the nitrided layer which is formed by diffusion of N inside of the steel material from the surfacemost layer, when formed without induction hardening, will be referred to as a “nitrided layer” and, when formed with induction hardening, will be referred to as a “hardened nitrided layer” so as to differentiate them.
The steel material which is produced by the method of production of PLT 1 is high in surface hardness, but is low in concentration of N in the hardened nitrided layer, so the hardness of the steel material at the time of a high temperature is low and it is not possible to obtain a sufficient softening resistance at the surface of gears etc. which become a high temperature during operation. As a result, it is not possible to obtain a high surface fatigue strength.
PLT 2 proposes a method of production which combines soft nitriding and induction hardening to obtain a part for machine structural use superior in mechanical properties. In the method of production of PLT 2, elements with a high affinity with N are added to the steel material so as to cause the nitrides in the steel material to break up and be diffused.
However, with the method of production of PLT 2, the amounts of addition of the elements for breaking up and diffusing the nitrides in the steel material are not sufficient, so it is necessary to heat the steel material to 900° C. to 1200° C., an extremely high temperature, by induction heating and make the N form a solid solution in the steel. For this reason, a thick oxide layer is formed at the steel material surface. Due to that oxide layer, the steel material is unavoidably remarkably degraded in mechanical properties.
Further, when converting the compound layer which is obtained by soft nitriding to a hardened nitrided layer by induction hardening, in the method of production of PLT 2, no thought is given to a method of increasing the thickness of the hardened nitrided layer.
Therefore, the part for machine structural use which is obtained by the method of production of PLT 2 is not sufficient in the thickness of the hardened nitrided layer, so does not have a surface fatigue strength good enough for use at a high surface pressure.
PLT 3 proposes the art of combining nitriding and induction hardening to obtain a part for machine structural use which has superior mechanical properties. The part for machine structural use of PLT 3 is obtained by nitriding a steel material at a 600° C. or more high temperature to form a compound layer, then performing induction hardening to form a hardened nitrided layer.
However, the nitriding in PLT 3 is performed at a 600° C. or more high temperature, so the compound layer which is formed is thin and the concentration of N in the compound layer is also low. Therefore, even if nitriding, then induction hardening, the nitrogen compounds in the compound layer which is formed by the nitriding are decomposed and the amount of N which diffuses to the inside of the steel material is small.
That is, with nitriding performed at a 600° C. or more high temperature, even if it is possible to form a compound layer, then perform induction hardening to form a hardened nitrided layer, the thickness of the hardened nitrided layer is insufficient, a sufficient softening resistance cannot be obtained, and as a result a part for machine structural use which has a good surface fatigue strength cannot be obtained.
PLT 4 proposes a method of production of a machine structure part comprising performing soft nitriding under conditions giving a nitrided layer depth of 150 μm or more, then performing induction hardening under conditions where the nitrided layer is transformed to austenite so as to thereby form a hardened nitrided layer.
However, the part for machine structural use which is produced by the method of production which is proposed in PLT 4 has a thickness of the hardened nitrided layer of 0.3 mm even at the maximum. The surface fatigue strength is not sufficient.
PLT 5 proposes a part for machine structural use which is obtained by heat treating a hot worked steel material for graphite precipitation, then cold working it and finally nitriding it.
However, the part for machine structural use of PLT 5 uses the precipitated graphite to improve the machineability. The precipitated graphite at the steel material surface causes the surface fatigue strength to drop.
Therefore, even if treating the part for machine structural use of PLT 5 by induction hardening to form a hardened nitrided layer, it is difficult to use the part for machine structural use of PLT 5 as a gear or other power transmission part of a transmission where a high surface pressure is applied to the surface of the part for machine structural use.
Further, in general, gears and other power transmission parts are obtained by forging, then machining the materials to finish them to the shapes of the parts, then surface hardening them to obtain the completed parts. The proposals in the above PLTs 1 to 5 are arts aimed at raising the strength of the operating surfaces by treating medium carbon steel containing alloy elements for surface hardening.
Therefore, since the machineability is not considered, the unnecessary rise in hardness at the inside of the steel material causes a drop in the productivity at the time of machining and therefore the manufacturing costs unavoidably rise.
Accordingly, it is desired to improve the surface fatigue strength of a steel material while keeping down a rise in hardness inside the steel material and preventing a drop in machineability.